The present invention relates generally to the art of metallurgy and, more particularly, to steel compositions and methods of processing that provide good grain-coarsening resistance and optimum core toughness in case-carburized steel components for automotive and machine structural applications.
Case-carburized components are utilized in a variety of automotive and machine structural components that are typically subjected to high cyclic stresses in service. Carburizing provides a hard, wear-resistant surface layer with good fatigue resistance. The resistance to fatigue is derived from (i) the inherent strength of the case microstructure, (ii) the presence of compressive residual stresses in the carburized case, and (iii) the suppression of intergranular fatigue crack initiation through control over the extent of quench embrittlement and intergranular oxidation, see "Microstructure and Performance of Carburized Steel, Parts I-IV," G. Krauss, Advanced Materials & Processes, 1995. Many components are also subjected to moderately frequent overloads in service, and under these conditions the mechanical properties of the underlying core material may become a factor in the overall durability of a component. The importance of core microstructure is exacerbated by the current trend in the automotive industry to increase the power density of highly stressed components via reductions in weight. These increases in power density are sometimes achieved by redesigning to minimize the decrease in component stiffness that typically accompanies reductions in weight. However, the ability to maintain component stiffness tends to be limited by geometric constraints in many mechanical systems, and in these cases structural integrity may exhibit a substantial dependence on the through-section properties of a component. Thus, improvements in the durability of more compliant components subjected to high nominal stresses and overloads will necessarily require carburizing grades of steel with improved toughness. Since high-carbon martensite exhibits low absolute levels of toughness, improvements in the through-section toughness of case-carburized components will primarily depend on the ability to generate substantially improved core toughness after carburizing at elevated temperatures for extended periods of time.
Air-melt, aluminum-killed steels comprise a vast majority of the "fine-grained" carburizing grades of steel in use today. Recent work on air-melt steels has shown that toughness is primarily limited by the presence of coarse grain-refining precipitates in lightly-tempered martensitic microstructures, see for example, M. J. Leap, U.S. Pat. No. 5,409,554. However, the mechanism of toughening associated with the refinement of coarse grain-refining precipitates is limited to comparatively short austenitizing times (i.e., times representative of reheating for the hardening operation). Austenitization for extended periods of time promotes precipitate coarsening at constant volume fraction, such that the toughness of aluminum-killed steels with refined dispersions of AlN precipitates is significantly degraded after austenitization under conditions representative of carburizing. For the case of conventionally-processed steels containing coarse grain-refining precipitates, austenitization for extended periods of time will at most provide a very small increase in toughness. This small increment of toughening is derived from the dissolution of intermediate-sized precipitates in the dispersion during the coarsening process. Thus, the prior art does not address the effects of precipitate coarsening on the development of toughness in tempered martensitic microstructures, particularly when a fine-grained microstructure is desired after austenitization at elevated temperatures for extended periods of time.